While reading Brown and Duguid's "Social Life of Information", I came across this brilliant example by Paul Duguid of witting and unwitting testimony (2002 edition, p173).
"I was working in an archive of a 250 year old business, reading correspondence from about the time of the American Revolution. Incoming letters were stored in wooden boxes about the size of a standard Styrofoam picnic cooler, each containing a fair portion of dust as old as the letters. As opening a letter triggered a brief asthma attack, I wore a scarf tied over my nose and mouth. Despite my bandit's attire, my nose ran, my eyes wept and I coughed, wheezed and snorted. I longed for a digital system that would hold the information from the letters and leave paper and dust behind.
"One afternoon another historiam came to work on a similar box. He read barely a word. Instead he picked out bundles of letters and, in a move that sent my sinuses into shock, ran each letter beneath his nose and took a deep breath, at times almost inhaling the letter itself but always getting a good dose of dust. Sometimes, after a particularly profound sniff, he would open the letter, glance at it briefly, make a note and move on.
"Choking behind my mask, I asked him what he was doing. He was, he told me, a medical historian. (A profession to avoid if you have asthma.) He was documenting outbreaks of cholera. When that disease occurred in a town in the eighteenth century, all letters from that town were disinfected with vinegar to prevent the disease from spreading. By sniffing for the faint traces of vinegar that survived 250 years and noting the date and source of the letters, he was able to chart the progress of cholera outbreaks.
"His research threw new light on the letters I was reading. Now cheery letters telling customers and creditors that all was well, business thriving and the future rosy read a little differently if a whiff of vinegar came off the page. Then the correspondent's cheeriness might be an act to prevent a collapse of business confidence - unaware that he or she would be betrayed by a scent of vinegar."